Fix GEO_NORMALIZE_DISABLE_TOWGS84 definition

Fix "missing template: GEO_NORMALIZE_DISABLE_TOWGS84" error by adding
a non-empty description of GEO_NORMALIZE_DISABLE_TOWGS84 in AC_DEFINE

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>

Index: libgeotiff/configure.ac
===================================================================
--- libgeotiff/configure.ac	(révision 2818)
+++ libgeotiff/configure.ac	(copie de travail)
@@ -317,7 +317,7 @@
 AM_CONDITIONAL([CSV_IS_CONFIG], [test ! x$CSV_CONFIG = xno])
 
 
-AC_ARG_ENABLE(towgs84, [  --disable-towgs84       Disable WGS84 parameters for binary compatibility with pre-1.4.1], AC_DEFINE(GEO_NORMALIZE_DISABLE_TOWGS84))
+AC_ARG_ENABLE(towgs84, [  --disable-towgs84       Disable WGS84 parameters for binary compatibility with pre-1.4.1], AC_DEFINE(GEO_NORMALIZE_DISABLE_TOWGS84, [], [Disable WGS84 parameters]))
 
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